In the prior art it is known that a differential voltage transformer may be employed, where an AC voltage signal is applied across the primary windings thereof and induces an equal voltage per turn across the secondary or coupled windings thereof. With ideal coupling, the voltage per turn from winding to winding is exceedingly well matched. This property gives rise to a less understood but widely used transformer application: the common-mode choke. In the common-mode choke, a differential pair of signals is applied to two same-phased poles of two windings to have their circuit completed through two oppositely-phased poles of the same two windings. If coupling between windings is perfect, the magnetic flux of one winding perfectly cancels that of the other, and the common-mode choke introduces no inductance whatsoever to the circuit. However, any voltage applied to one set of like-phased poles with respect to their oppositely-phased pair appears therebetween, with the result being that both windings drop precisely the same voltage in precisely the same phase. Thus, a common-mode choke can AC level-shift a differential AC or DC signal, as is known. As is also known, a common-mode choke is not limited to two windings but instead may be made with many windings to AC level-shift many circuits.
Though differential currents generate in the common-mode choke no flux, the same is not true of common-mode currents. Any common-mode voltage causes inductive common-mode current flow through the common-mode choke as it would in any inductor. As common-mode frequency approaches zero, and as inductive reactance disappears, inductive voltage drop across any winding proportionally vanishes and the common-mode choke becomes inutile as a level shifter. A DC common-mode voltage may cause a large common-mode current, and may saturate any common-mode choke core rendering it inoperative for AC as well.
Accordingly, a need exists for a method and mechanism to avoid saturating a common-mode choke core when a DC common-mode voltage is applied thereto. Moreover, a need exists for such a method and mechanism where a common-mode level shifter can shift wideband DC differential signals across large AC or DC common-mode voltage differences with exceptional fidelity, even in the presence of common-mode transients, and where a sampling bridge faithfully acquires signals in the presence of severe common-mode disturbances.